The use of iron oxide in soda-lime glass systems for improved UV absorption is known. In such systems the iron oxide content has been up to 0.12 weight %. In these systems, however, this dopant level is known to reduce visible transmission of the glass, particularly in the 650-750 nm range, by 1-2% with glass thickness of 1 mm, an objectionable feature.
UV absorbing borosilicate glass compositions for electric lamps such as high intensity discharge lamps, typically contain either lead and arsenic oxides or lead and cerium oxides. Both lead and arsenic are toxic materials and it would be very advantageous to be able to manufacture acceptable glasses without using these materials. Arsenic oxide is generally employed in glass compositions as a fining agent for glasses which are difficult to fine (i.e., removal of bubbles). Ceria has been used as an acceptable substitute for arsenic oxide for fining glasses. However, in order to use ceria-containing borosilicate glass compositions in the manufacture of outer jackets for high intensity discharge lamps, it has been found necessary to include lead oxide to prevent solarization of the ceria. Absent the lead, it has been found that ceria will produce light absorbing color centers upon UV irradiation. These light absorbing centers darken the outer jacket and seriously reduce the light output. Elimination of the lead and arsenic oxides has produced workable glasses with adequate sealing capabilities to tungsten; however, such glasses (for example, Schott 8487, which is used in Europe for lamp stems and tubulations) do not absorb sufficiently in the UV region to be used as outer envelopes, thus necessitating two types of glass, one for the stem and tubulation and one for the outer jacket. As an illustration, the Schott glass enumerated above has a transmittance of 23% @300 nm, whereas open fixtured lamps employed in the United States must meet the requirements of UL1572 Specification which requires a transmittance of no more than 8% @300 nm. Additionally, to be an acceptable glass for use as the outer jacket of high intensity discharge lamps, any absorption at wavelengths approaching the visible, say, above about 375 nm, must be minimized.